Pressure vessel heads are fastened to the vessel body with an intermediate gasket by studs and nuts arranged in a bolting circle. It is essential that each nut be screwed with precisely the same torque to ensure a perfectly-tight closing of the head.
In the past, it was very difficult and time-consuming to obtain a leak-proof joint, even though a torque-metering wrench was used for screwing the nuts, because the latter were screwed one after the other, resulting in unequal flattening of the gasket around the head. This problem frequently occurs in nuclear pressure vessels, such as heat exchangers, wherein the gasket is metallic and must be replaced when unevenly flattened.